The conventional plastic molding machines, either of the extrusion or of the injection type, normally are fed from a main feed hopper which contains a mixture of a primary resinous material and a color concentrate or other additive material. The primary and additive materials are premixed by placing the two materials in a barrel which is then tumbled for mixing the materials together. Thereafter the mixed materials are loaded into the main feed hopper of the molding machine. However, as the mixture is allowed to stand in the main feed hopper, the color powder tends to sift to the bottom of the hopper through the bulkier and larger resinous granule materials causing inaccuracies in the coloring of the final product and waste of materials.
Buying bulk quantities of uncolored resinous plastic material and then blending in the color concentrate or color powder is desirable since a sizeable cost savings is afforded over buying resinous materials already colored. Therefore, a more efficient and accurate method and apparatus for mixing the primary resinous material with the additive coloring material needs to be provided. Prior devices have been developed for feeding a primary material and an additive from separate hoppers into a common chamber for mixing, such as in U.S. Pat. Nos. 3,164,860 and 2,382,655 wherein a primary molding material and an additive material are fed from separate hoppers to a conveying chamber and are mixed therein by mixing elements carried on the screw conveyors, or are otherwise caused to be mixed together by the screw conveyor itself.
Another device is shown in U.S. Pat. No. 2,442,368 wherein a frictional mixing chamber is provided below the main hopper for plasticizing the plastic material to be molded which also makes it possible to mix ingredients into the plastic material at the time of plasticizing thus avoiding premixing.